To: atm{at}shore.net
From: "James W. Burrows" 
Reply-To: "James W. Burrows" 


There's a notorious footnote in Texereau, p. 83, that has the above quote
relative to the choice of the representative zone radii to be used for
fixed-source Foucault with a Couder mask.  Also, there's the long-standing
debate about whether the Foucault test will say a perfect mirror is under-
or over-corrected.

Diffract shows the Couder zone contrast difference for a given longitudinal
setting.  A short fiddle will give the 0-contrast difference setting for
each zone.  The results for Texereau's "standard mirror", 200 mm
f/6, are:

Hx              Hm (ave)        Hn (rms)        (Nils Olof)     D
0
50              25              35.36           30.18           0.487
70.7            60.35           61.23           60.79           1.508
86.6            78.65           79.05           78.85           2.588
100             93.3            93.54           93.42           3.661

RMS, nm    3.2          1.4             1.8
b               -.966           -1.019          -.987

D are the theoretical readings from Diffract, and Nils Olof recommends the
ave(ave, rms) zone radii.  It turns out that Couder's rms wins out in this
case, although Nils' choice is better for more narrower zones.

Yes, Virginia, Foucault does say a perfect mirror is a tiny bit
undercorrected, if you use Hm. Tex agrees, showing (very small) high center
and edge for the above readings.

         -- Jim Burrows
         --              mailto:burrjaw{at}earthlink.net
         --              http://home.earthlink.net/~burrjaw
         -- Seattle      N47.47233, W122.36620 (WGS84)

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